The Rolls-Royce Pegasus, formerly the Bristol Siddeley Pegasus, is a turbofan engine originally designed by Bristol Siddeley, and was manufactured by Rolls-Royce plc. This engine is able to direct thrust downwards which can then be swivelled to power a jet aircraft forward.[1] Lightly loaded, it can also manoeuvre like a helicopter, vertically for takeoff and landings.[2] In US service, the engine is designated F402.

The unique Pegasus engine powers all versions of the Harrier family of multi-role military aircraft. Rolls-Royce licensed Pratt & Whitney to build the Pegasus for US built versions. However Pratt & Whitney never completed any engines, with all new build being manufactured by Rolls-Royce in Bristol, England. The Pegasus was also the planned engine for a number of aircraft projects, among which were the prototypes of the German Dornier Do 31 VSTOL military transport project.[3]

Michel Wibault, the French aircraft designer, had the idea to use vectored thrust for vertical take-off aircraft. This thrust came from four centrifugal compressors driven by a Bristol Orionturboprop, the exhaust from each could be directed by rotating the outlets.[4]Gordon Lewis initially planned an engine with two thrust vectors, driven by the compressor, with forward thrust from a conventional rear exhaust in his initial BE.52 design. The BE.52 design was built around a Bristol Siddeley Orpheus which through a shaft drove the first three stages of a Bristol Olympus engine which had inlet and outlets separate of those of the Orpheus. Work was overseen by Bristol Siddeley's Technical Director Stanley Hooker.

The Bristol Engine Company began work on the BE.53 Pegasus in 1957. While the BE.52 was a self-contained powerplant and lighter compared to Wibault's concept, the BE.52 was still complicated and heavy. In the BE.53 the Olympus stages were fitted close to the Orpheus stage; this simplified the inlet ducting and the Olympus stages now supercharged the Orpheus improving the compression ratio.[5]

The engine was designed in isolation for a year, then it was helped greatly by understanding what type of aircraft it was designed for. The team received a supportive letter from Sydney Camm of Hawker in May 1957. Hawker were looking for a Hawker Hunter replacement. The aircraft designer, Ralph Hooper, suggested having the four thrust vectors (originally suggested by Lewis), with hot gases from the rear two. Two thrust vectors did not provide enough lift.[citation needed] The 1957 Defence White Paper, which focused on missiles, and not manned aircraft - which were declared 'obsolete', also was not good news, because it precluded any future government financial support for development of non-already extant manned combat aircraft, and so prevented any official financial support for the engine/airframe from the Ministry of Defence.[6]

The further development of the engine then proceeded in tandem with the airframe the Hawker P.1127, which first flew in 1960. The next stage of design and development was then flown in the Kestrel, of which nine were built. This was then developed into the Harrier combat aircraft. The engine was financially supported to the tune of 75% from the Mutual Weapons Development Programme, Verdon Smith of Bristol Siddeley Engines Limited (BSEL), which Bristol Engines had by then become on its merger with Armstrong Siddeley, quickly agreeing to pay the remainder.[6]

The flight testing and engine development received no government funding; the plane's funding came entirely from Hawker. There was only enough thrust in the first engines to barely lift the plane off the ground because of weight growth problems. Flight tests were conducted when the aircraft was tethered. The first free hover was achieved on 19 November 1960. The first, and difficult, transition from static hover to conventional flight was achieved on 8 September 1961. The RAF was not much of a convert to the VTOL idea, and described the whole project as a toy and a crowd pleaser. The first prototype P1127 made a very heavy landing at the Paris Air Show in 1963.

Series Manufacture and Design and Development improvement to ever higher thrusts of the Pegasus was continued by Bristol engines beyond 1966, which was when Rolls-Royce Ltd bought the Company. A related engine design, the 39,500 lbf (with reheat) Bristol Siddeley BS100 for a supersonic VTOL fighter (the Hawker Siddeley P.1154) was not developed to production as the aircraft project was cancelled in 1965.

A non-vectored 26,000 lb thrust derivative of the Pegasus running on liquid hydrogen, the RB.420, was designed and offered in 1970 in response to a NASA requirement for an engine to power the projected Space Shuttle on its return flight through the atmosphere. In the event, NASA chose a design using an un-powered gliding return for the resultant vehicle.[7]

The Pegasus vectored-thrust turbofan is a two-shaft design featuring three low pressure (LP) and eight high pressure (HP) compressor stages driven by two LP and two HP turbine stages respectively. Unusually the LP and HP spools rotate in opposite directions to greatly reduce the gyroscopic effects which would otherwise hamper low speed handling. LP and HP fan blading is titanium, the LP fan blades operating in the partly supersonic region, and airflow is 432 lb/s.[6] The engine employs a simple thrust vectoring system that uses four swiveling nozzles, giving the Harrier thrust both for lift and forward propulsion, allowing for STOVL flight.

Combustion system is an annular combustor with ASM low-pressure vaporising burners.[6]

The front two nozzles, which are of steel, are fed with air from the LP compressor, the rear nozzles, which are of Nimonic[6] with hot (650 °C) jet exhaust. The airflow split is about 60/40 front back.[8] It was critical that the nozzles rotate together. This was achieved by using a pair of air motors fed from the HP (high pressure) compressor, in a fail over configuration, pairs of nozzles connected with, surprisingly, motor-cycle chains. The nozzles rotate over an angular range of 98.5 degrees.[6]

The Pegasus was also the first turbofan engine to have the initial compressor fan, the zero stage, ahead of the front bearing. This eliminated radial struts and the icing hazard they represent.

The engine is mounted in the centre of the Harrier and as such it is necessary to remove the wing to change the powerplant having already sat the fuselage on trestles; the whole change took a minimum of eight hours[9] although using the proper tools and lifting equipment this could be accomplished in less than four.[10]

The maximum take-off thrust available from the Pegasus engine is limited, particularly at the higher ambient temperatures, by the turbine blade temperature. As this temperature cannot reliably be measured, the operating limits are determined by jet pipe temperature. To enable the engine speed and hence thrust to be increased for take-off, water is sprayed into the combustion chamber and turbine to keep the blade temperature down to an acceptable level.

Water for the injection system is contained in a tank located between the bifurcated section of the rear (hot) exhaust duct. The tank contains up to 500 lb (227 kg, 50 imperial gallons) of distilled water. Water flow rate for the required turbine temperature reduction is approximately 35gpm (imperial gallons per minute) for a maximum duration of approximately 90 seconds. The quantity of water carried is sufficient for and appropriate to the particular operational role of the aircraft.

Selection of water injection engine ratings (Lift Wet/Short Lift Wet) results in an increase in the engine speed and jet pipe temperature limits beyond the respective dry (non-injected) ratings (Lift Dry/Short Lift Dry). Upon exhausting the available water supply in the tank, the limits are reset to the 'dry' levels. A warning light in the cockpit provides advance warning of water depletion to the pilot.

The 11-21 was developed for the second generation Harriers, the USMC AV-8B Harrier II and the BAE Harrier IIs. The original model provided an extra 450 lbf (2.0 kN). The RAF Harriers entered service with the 11-21 Mk.105, the AV-8Bs with F402-RR-406. Depending on time constraints and water injection, between 14,450 lbf (64.3 kN) (max. continuous at 91% RPM) and 21,550 lbf (95.9 kN) (15 s wet at 107% RPM) of lift is available at sea level (including splay loss at 90°).[11]

The Mk.106 development was produced for the Sea Harrier FA2 upgrade and generates 21,750 lbf (96.7 kN).

The 11-61 (aka -408) is the latest and most powerful version of the Pegasus, providing 23,800 lbf (106 kN).[12] This equates to up to 15 percent more thrust at high ambient temperatures, allowing upgraded Harriers to return to an aircraft carrier without having to dump any unused weapons which along with the reduced maintenance reduces total cost of engine use.

This latest Pegasus has also enabled a highly effective radar equipped AV-8B+. This version combines the proven advantages of day and night STOVL operations with an advanced radar system and beyond-visual-range missiles. The RAF/RN was in the process of upgrading its GR7 fleet to GR9 standard, initially through the Joint Upgrade and Maintenance Programme (JUMP) and then through the Harrier Platform Availability Contract (HPAC). All GR7 aircraft were expected to have been upgraded by April 2010.[needs update] Part of this process was the upgrade of the Mk.105 engines to Mk.107 standard. These aircraft were known as GR7As and GR9As.